It is well known to provide within a building or similar enclosure a plurality of image capture devices mounted or otherwise placed at various key locations so as to provide a surveillance system for monitoring and/or capturing unauthorised activities within the building or enclosure.
In its simplest form, such a surveillance system comprises a plurality of image capture devices, such as cameras for capturing still or (more commonly) moving images, each of which is linked or connected to a respective display screen or monitor on which the images captured by the devices can be viewed. Thus, a typical system would have a plurality of monitors located at a central position for simultaneous viewing of the images captured by each of a plurality of image capture devices.
However, such systems are somewhat inefficient in the sense that many (if not all) of the images captured by the devices will be of little or no interest to the viewer for much of the time as no unauthorised activities will be taking place within their respective fields of view. In fact, it is most likely that unauthorised activities, if any, will be taking place within the field of view of only a single image capture device at any one time, making the rest of the image capture devices (and their respective monitors or screens) redundant. Further, it can be quite difficult for a single viewer to monitor the content of images being displayed on several different monitors at the same time, thereby giving rise to the possibility that any unauthorised activities would be missed.
In another known system, a plurality of cameras are linked or connected to a single monitor, the monitor displaying the images captured by each of the cameras for a predetermined time in a predetermined order. This is obviously undesirable as any unauthorised activities may be taking place within the field of view not currently being displayed on the monitor or screen, again increasing the possibility that such activities will go undetected. A variation of this type of system comprises a plurality of cameras being connected to a single viewing means, such viewing means being arranged to monitor incoming images, detect movement therein and display the images from the device within whose field of view the movement is detected. However, the continuous transmission of images captured by a plurality of image capture devices requires a transmission bandwidth, storage capacity and power consumption which is in many cases unacceptably high.
U.S. Pat. No. 5,602,585 describes a motion detection camera system which may be used as a security camera and includes an active pixel imaging system operable in a video and differential mode connected to a mode activation circuit that controls the imaging system mode. In the video mode, the imaging system generates an output signal containing information of a viewed area in a frame-by-frame manner. In the differential mode, the imaging system generates a differential output signal containing information consisting of the differences in the viewed area between adjacent detected frames. Normally, the imaging system camera is operated in the differential mode. If no motion occurs in the viewed area, the corresponding differential output signal representing the interframe differences will have a constant signal level. Upon detecting that the differential output signal deviates by greater than a threshold level, thereby indicating motion in the viewed area, the mode activation circuit causes the imaging system to operate in its video mode. The corresponding generated video output signal of the imaging system is then provided to monitoring equipment, such as a video cassette recorder for recording of the motion in the viewed area.
Thus, in the type of system described above, video images are only transmitted to the monitoring equipment if motion is detected within the field of view of the camera. However, as the motion detection circuitry is included within the camera, it can only control the resultant output from that camera and, as such, for a plurality of cameras in a multi-camera surveillance system, individual motion detection and mode activation circuits would have to be provided for each camera. Further, if the movement is taking place between the fields of view of a plurality of cameras, any time delay between the movement being detected and the respective camera being arranged to operate in its video mode would cause at least some, if not all, of the apparent activity to go undetected.
European Patent Application Number EP-A-0979009 describes a camera which normally transmits captured images to a viewing area in the form of low resolution (or highly compressed) image data. At the same time, a high resolution (i.e. less compressed) for of the image data is stored locally by the camera. The camera includes a control unit which can be used to monitor captured images and detect any unusual conditions therein, for example, motion of an object or entity, and transmit a signal to the viewing area alerting the user of the occurrence of such a condition. The user can, if required, retrieve the high resolution data for viewing upon request. Thus, the camera is always generating two sets of image data: a low resolution version for transmission across a wireless link, and a high resolution version for local storage and retrieval for viewing at a later time, if requested manually by the user. The generation of two sets of data representing the same images is obviously inefficient, and often unnecessary in the sense that the high resolution data is only likely to be required for viewing in the event of an unusual condition being detected. Further, the high resolution image data cannot be viewed in real time.
We have now devised an arrangement which overcomes the problems outlined above.